1. Field of the Invention
The present invention relates to an auto-tensioner for maintaining the tension in a belt at a constant level. More particularly, the present invention is concerned with a multi-disk fluid viscosity type auto-tensioner capable of stably maintaining a constant tension in a driving belt used in an automotive engine, such as a timing belt for driving a cam shaft or cam shafts in an overhead cam shaft (OHC) or dual overhead cam shaft (DOHC) engine or an auxiliary machine drive belt in such an engine.
2. Related Background Art
The auto-tensioner of the invention finds a wide use in various machines having driving belts which are required to be tensed at a constant level. The following description, however, will be focused on a toothed drive belt used in an OHC or a DOHC automotive engine wound around toothed pulleys mounted on the engine block. Toothed drive belts are known which are used for transmitting power between to toothed pulleys which are mounted on a cylinder block, such as, for example, a timing belt for driving a cam shaft or cam shafts in an OHC or DOHC engine stretched between timing pulleys. Usually, a certain level of initial tension is imparted by a fixed-type tensioner to such a belt in order to prevent slip of the toothed belt and the timing pulley. In modern automotive engines having engine blocks made of aluminum, for example, a problem arises due to a difference in the thermal expansion coefficient between the aluminum engine block and the belt which is made of a rubber reinforced with a metallic wire. The aluminum engine block has a thermal expansion coefficient which is about four times as large as that of the belt, so that the level of the tension in the belt is largely changed by a change in the temperature. Namely, when the temperature is low, the distance between the pulleys is decreased by an amount greater than the amount of contraction of the belt so that the belt becomes loose, with the result that the belt vibrates largely or slips on the pulleys. Conversely, when the temperature is high, the distance between the pulleys is increased by an amount greater than the amount of thermal expansion of the belt, so that the belt is excessively tensed to increase the level of noise or to adversely affect the durability of the belt.
In order to overcome these problems encountered by the fixed-type tensioner, there have been proposed tensioners having a function for automatically compensating any change in the tension as disclosed, for example, in Japanese Patent Laid-Open Nos. 208343/1982 and 49054/1984.
These proposed tensioners have an elastic damping material placed between the rotary part rotatable in contact with the belt and the stationary part fixed to the engine block, the elastic vibration damper material being compressed when the tension in the belt tends to increase, thereby maintaining the tension at a moderate level.
In this type of tensioner, the elastic damping material is always loaded by the tension in the belt so that it is plastically deformed over time. The plastically deformed vibration damper material can no longer compensate for loosening of the belt and allows problems such as vibration of belt and slip of the belt to occur.